Set accelerator for gypsum hydration

Abstract

In a first embodiment of the invention, aged brushite is combined with calcium sulfate hemihydrate and water to make a gypsum slurry. Aged brushite slurry behaves similarly to calcium sulfate dihydrate to act as a seed crystal and rapidly initiate crystallization. The brushite slurry does not require the addition of a coating to maintain its activity over time compared to calcium sulfate dihydrate set accelerators. A gypsum-based product made using brushite slurry as the set accelerator is a second embodiment. The product has the brushite molecules integrated as part of the calcium sulfate dihydrate matrix and is distributed throughout the matrix.

1. A gypsum product comprising:
 a matrix of calcium sulfate dihydrate crystals with 
 ground brushite crystals distributed throughout said matrix, wherein said ground brushite crystals are present in amounts of about 0.013% to about 1.7% by weight of the ground brushite by weight of the calcium sulfate dihydrate. 
 
     
     
       2. The gypsum product of  claim 1  wherein said ground brushite is uncoated. 
     
     
       3. The gypsum product of  claim 1  wherein said ground brushite has a particle size distribution of about 1 micron to about 20 microns. 
     
     
       4. The product of  claim 1  wherein said ground brushite is aged in a slurry with water more than 48 hours after grinding. 
     
     
       5. A method of making a gypsum article comprising:
 obtaining calcium sulfate hemihydrate; 
 obtaining a slurry of ground brushite, wherein said ground brushite is present in amounts of about 0.015% to about 2% by weight dry ground brushite by weight of the dry calcium sulfate hemihydrate; 
 mixing the brushite slurry, calcium sulfate hemihydrate and water to form a gypsum slurry; 
 forming a product from said gypsum slurry; and 
 allowing the gypsum slurry to set. 
 
     
     
       6. The method of  claim 5  further comprising aging said slurry of ground brushite to form an aged brushite slurry prior to said mixing step. 
     
     
       7. The method of  claim 5  wherein said ground brushite of said obtaining step has a particle size of less than about 40 microns. 
     
     
       8. The method of  claim 7  wherein said ground brushite of said obtaining step has a particle size of about 1 micron to about 20 microns. 
     
     
       9. The method of  claim 5  wherein said forming step comprises depositing said gypsum slurry on a facing and shaping the gypsum slurry to form a panel.